Technical Field
The present disclosure generally relates to strained channel FinFET devices and, in particular, to a strained channel FinFET in which the channel incorporates two different semiconductor materials.
Description of the Related Art
Strained silicon transistors have been developed to increase mobility of charge carriers, i.e., electrons or holes, passing through a semiconductor lattice. Introducing tensile stress into an n-FET transistor tends to increase electron mobility in the channel region, resulting in a faster switching response to changes in voltage applied to the transistor gate. Likewise, introducing compressive stress into a p-FET transistor tends to increase hole mobility in the channel region, resulting in a faster switching response. Various methods can be used to introduce tensile or compressive stress into transistors, for both planar devices and FinFETs.
One way to introduce strain is to replace bulk silicon from the source and drain regions of the substrate, or from the channel itself, with silicon compounds such as silicon germanium (SiGe), for example. Because germanium-silicon bonds are longer than silicon-silicon bonds, there is more open space in a SiGe lattice. The presence of germanium atoms having longer bonds stretches the lattice, causing internal strain. Electrons can move more freely through a lattice that contains elongated Ge—Ge bonds, than through a lattice that contains shorter Si—Si bonds. Replacing silicon atoms with SiGe atoms can be accomplished during a controlled process of epitaxial crystal growth, in which a new SiGe crystal layer is grown from the surface of a bulk silicon crystal, while maintaining the same crystal structure of the underlying bulk silicon crystal.
Strain and mobility effects, thus, can be tuned by controlling the elemental composition within the epitaxially grown crystal. For example, it has been determined that epitaxial SiGe films containing a high concentration of germanium, e.g., in the range of 25%-55%, provide enhanced hole mobility compared with lower concentration SiGe films. Thus, it is advantageous to increase the percent concentration of germanium atoms in the fins in a FinFET. However, the lattice structures of high germanium concentration films tend to be mechanically unstable, especially if they contain a high number of dislocation type defects.